1. create your curve as an image using your own choice of graphing/plotting software, and add the image to your docx (this is probably the simplest, and you can also export your docx to PDF and HTML directly from docx4j)

2. draw it using DrawingML or VML primitives (much as you would if you were to draw a curve in SVG)

3. use Word's chart features. You end up with an embedded xlsx, a part c:chartSpace where xmlns:c="http://schemas.openxmlformats.org/drawingml/2006/chart", and something like this in your main document part: